Does genetic biocontrol always use organisms that are genetically modified?

This is a matter of definition. If one defines genetic modification as a change in the genetic material through the use of modern biotechnology (genetic engineering), then the answer is no. Genetic biocontrol does not always use organisms that are genetically modified. The genetic make-up of an organism can be altered in several ways other than via molecular biology. Traditionally, this has been accomplished over time through selective breeding. Genetic changes also can be introduced using irradiation, as is the case for the classical Sterile Insect Technique (SIT), or by infection of the organism with a new microbe, such as a virus or bacterium.

There are variants of SIT and other biocontrol strategies that involve the release of insects that have been modified in the laboratory (genetically engineered) to effect a functional change. Genetic changes introduced using molecular biology technologies are expected to be more controllable and predictable than the random chromosomal damage caused by irradiation.

Fore more information:

https://royalsociety.org/topics-policy/projects/gm-plants/how-does-gm-differ-from-conventional-plant-breeding/